1. Field of the Invention
The invention relates to a method and arrangement for detecting a watermark in a compressed video signal. The invention also relates to an arrangement for decoding a compressed video signal so as to obtain a signal suitable for watermark detection.
2. Description of the Related Art
Watermarking is a technique of embedding imperceptible information in multimedia contents, such as, audio, still images or moving video. Watermarks are used for applications, such as, ownership verification, copyright protection and copy and playback control.
A watermark is often embedded in a video signal by slightly modifying the luminance pixels of the video signal in accordance with a watermark pattern. For the purpose of understanding this invention, it suffices to imagine the watermark pattern as an array of +1 and −1 values added to an equally sized array of pixels. The array of pixels having the same size as the watermark pattern is hereinafter referred to as a “picture”. A picture may be a full-size video image (480*720 pixels for NTSC or 576*720 pixels for PAL) or a part thereof, for example, a sub-image of 128*128 pixels. If the watermark pattern is smaller than the image, it is known as a “tile”. The pattern is then repeatedly used to obtain a “tiled” image. It is assumed that a plurality of pictures is watermarked with the same watermark pattern.
Detection of a watermark in a picture is, in essence, a thresholded correlation operation. A watermark detector decides whether or not a suspect picture is watermarked by computing the amount of correlation between the suspect picture and the watermark pattern to be detected, and comparing the result with a predetermined threshold. An example of such a watermark detector is disclosed in Applicant's International Patent Application WO-A-98/03014, corresponding to U.S. Pat. No. 5,933,798.
The subject invention addresses the problem of detecting a watermark in a compressed video signal. Video compression reduces the amount of data to be transmitted or recorded. A well-known example is MPEG compression. Briefly summarized, MPEG compression includes discrete cosine transform (DCT) of blocks of pixel values into blocks of coefficients. The coefficients are quantized causing many coefficients to assume the value zero. The quantized coefficients are variable-length encoded by assigning a Huffman codeword to each run of zero coefficients and a subsequent non-zero coefficient. The pictures can be encoded autonomously (I-pictures), or predictively (P- and B-pictures). In the latter case, residual pixel blocks (which are left after subtracting motion-compensated prediction blocks) are transformed rather than the pixel blocks themselves.
A straightforward method of detecting the watermark employs a cascade arrangement of a conventional MPEG decoder and a conventional watermark detector. However, it has a total complexity which is too large to serve as a viable solution for mere watermark detection, because MPEG decoding is a costly operation in terms of numbers of operations, complexity and amount of memory. This is particularly true for a DVD drive which is envisaged to include a watermark detector so as to determine whether a video program may or may not be copied, but does not itself include an MPEG decoder.